class T::Private::Methods::DeclBuilder

def returns(type)
  check_live!
  if decl.returns.is_a?(T::Private::Types::Void)
    raise BuilderError.new("You can't call .returns after calling .void.")
  end
  if !decl.returns.equal?(ARG_NOT_PROVIDED)
    raise BuilderError.new("You can't call .returns multiple times in a signature.")
  end
  decl.returns = type
  self
end